Plug-type heat flux gauge

ABSTRACT

A plug-type heat flux gauge formed in a material specimen and having a thermoplug integrally formed in the material specimen, and a method for making the same. The thermoplug is surrounded by a concentric annulus, through which thermocouple wires are routed. The end of each thermocouple wire is welded to the thermoplug, with each thermocouple wire welded at a different location along the length of the thermoplug. The thermoplug and concentric annulus may be formed in the material specimen by electrical discharge machining and trepanning procedures.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the U.S.Government and may be manufactured and used by or for the Government forgovernmental purposes without the payment of any royalties thereon ortherefore.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to heat flux gauges and methodsof producing heat flux gauges, and more particularly to plug-type heatflux gauges having thermoplugs that are integrally formed into aspecimen material and methods for producing same.

2. Description of the Related Art

Recently, there has been increasing need for small, reliable, accurateand easily built heat flux gauges. For example, heat flux gauges areneeded to obtain transient and steady-state surface heat flux data forverification of models relating to heat transfer, durability andaerodynamics. In many applications, heat flux gauges are required to beaccurate over wide ranges of heat flux values that vary over widetemperature ranges. For example, in order to obtain rapid transientsurface heat flux measurements on turbine blades utilized in spacecraft,a heat flux gauge may be required to provide accurate measurements forsurface heat flux values varying from about 0.3 to 6 MW/m² over atemperature range of 100 to 1200K.

Water-cooled heat flux gauges are available for measurement to about 10MW/m², but only at surface temperatures that are maintained between 280to 360K. Also, water-cooled heat flux gauges tend to be relative large,e.g., often greater than 2.0 cm in diameter and length. Thus, because oftheir relatively large size and narrow temperature range, water-cooledheat flux gauges often cannot be used.

Another type of heat flux gauge is the plug-type heat flux gauge.Typically, plug-type heat flux gauges include a thermoplug that isscrewed or welded into a specimen material having an active surface,creating a seam between the active surface and the thermoplug. This seamcan introduce large discontinuities in surface temperature, resulting invery large errors in heat flux measurement. It is extremely difficult todetermine the uncertainty in the resulting heat flux measurements due tothe seam. Thus, the lack of accuracy and precision of conventionalplug-type heat flux gauges may prohibit their use.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved plug-type heatflux gauge that is accurate over wide ranges of heat flux values thatvary over wide temperature ranges.

Another object of the invention is to provide an improved plug-type heatflux gauge that is accurate, reliable and which may be small in size.

Still another object of the invention is to provide an improvedplug-type heat flux gauge that accurately measures transient andsteady-state surface heat flux.

Yet another object of the invention is to provide a method for easilyproducing an improved plug-type heat flux gauge that achieves theforegoing objects.

In order to achieve the foregoing and other objects, in accordance withthe purposes of the present invention as described herein, a thermoplugis formed in a specimen material using, for example, electricaldischarge machining and trepanning procedures. The machining produces anannulus that surrounds an integral thermoplug. Because the thermoplug isan integral part of the specimen material, there is no seam between thethermal plug and the active surface of the specimen material. Therefore,the very large errors in heat flux measurement due to the seam presentin prior art plug-type heat flux gauges do not occur in the presentinvention. Also, there is no need to determine the uncertainty of heatflux measurement caused by the presence of such a seam.

The ends of thermocouple wires are welded along the length of thethermoplug in order to form hot junctions. The thermocouple wires arerouted through the annulus to the rear of the specimen material, wherethey may be connected to larger diameter leadwire assemblies. A backcover may be welded to the back wall of the specimen material, therebyenclosing the thermoplug and annulus and trapping a thermal insulator,e.g., air, within the annulus and behind the thermoplug.

In a preferred method of making the invention, electrical dischargemachining is used to intermittently discharge an electric spark througha gap between the specimen material and an electrode, which includes ahole along its length. The specimen material and the electrode areimmersed in a dielectric fluid, and as current is applied, sparks areemitted from the end of the electrode tube, detaching portions of thespecimen material. The thermoplug is formed within the hole of theelectrode, as the electrode descends into the specimen material, therebyforming the thermoplug as an integral and unitary part of the specimenmaterial.

These and other features and advantages of the present invention willbecome more apparent with reference to the following detaileddescription and drawings. However, the drawings and description aremerely illustrative in nature and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate several aspects of the presentinvention, and together with the descriptions serve to explain theprinciples of the inventions. In the drawings:

FIG. 1 is an overall view of the plug-type heat flux gauge according tothe present invention; and

FIG. 2 is a graph that shows relative agreement between the heat fluxvalues obtained from the plug type heat flux gauge of the presentinvention and conventional heat flux gauges.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an overall view of a plug-type heat flux gauge 1 accordingto the present invention. A thermoplug 2 is integral with materialspecimen 3. Material specimen 3 includes front surface 4 (lower surfacein FIG. 1) and back surface 5 (upper surface in FIG. 1). Front surface 4of material specimen 3 is exposed to an energy source, e.g., an arc-lampfor radiating incident thermal radiation 6. Thermoplug 2 is insulated onall surfaces except front surface 4, and therefore heat transfer withinthermoplug 2 may be assumed 1-dimensional. Concentric annulus 7surrounds the sides of thermoplug 2. Because concentric annulus 7 isformed only partially through material specimen 3, a floor 8 ofconcentric annulus 7 is formed a finite distance from front surface 4 ofmaterial specimen 3. Concentric annulus 7 is formed into materialspecimen 3 using any appropriate method, e.g., trepanning, electricaldischarge machining or the like. Trepanning consists of machining acircular groove into a material. For example, a hole saw may be appliedto back surface 5 of material specimen 3 in order to form concentricannulus 7 and to form thermoplug 2 integral with material specimen 3.The length of thermoplug 2 is taken as the distance from front surface 4of material specimen 3 to a back surface 9 of thermoplug 2.

Since thermoplug 2 is an integral part of material specimen 3, there isno seam connecting thermoplug 2 to material specimen 3. As discussedabove, conventional plug-type heat flux gauges include such seams, whichcan cause large errors in heat flux measurement. Further, determinationof the uncertainty in heat flux measurement caused by such seams is verydifficult. Therefore, a plug-type heat flux gauge according to thepresent invention is advantageous in that there is no such seam. In apreferred method of forming concentric annulus 7 into material specimen3, a process known as electrical discharge machining is used. Electricaldischarge machining is well known in the machining art. One example ofan electric discharge machining apparatus is described in U.S. Pat. No.2,818,490, issued to Dixon et al. Thermoplug 2 and concentric annulus 7are formed by intermittently discharging an electric spark through a gapbetween material specimen 3 and an electrode. Material specimen 3 andthe electrode are immersed in a dielectric fluid, e.g., refinedkerosene. The electrode, which is made from a conductor, e.g.,copper-tungsten, includes a hole along its length. Sparks are emittedfrom the electrode and strike material specimen 3, thereby detachingmaterial. The electrode forms thermoplug 2 and concentric annulus 7 asthe electrode descends into material specimen 3, i.e., thermoplug 2 isformed within the hole of the electrode. Concentric annulus 7 ismachined only part way through material specimen 3, thereby formingthermoplug 2 as an integral part of material specimen 3. Thermoplugs ofall sizes and shapes may be formed using this process. For example,thermoplug diameters of 0.1 to 0.2 cm, thermoplug lengths of 0.1 to 0.3cm and concentric annulus widths of 0.03 to 0.08 cm have been formedusing electric discharge machining. Thus, plug-type heat flux gauge 1according to the present invention may be made relatively small comparedwith the size of conventional water-cooled heat flux gauges.Consequently, plug-type heat flux gauge 1 according to the presentinvention may be utilized in measuring heat flux in extremely smallmaterial specimens.

As shown in FIG. 1, a back cover 10 encloses thermoplug 2 and concentricannulus 7. Back cover 10 is secured, e.g., welded, to back surface 5 ofmaterial specimen 3, thereby trapping a thermal insulator, e.g., air,within concentric annulus 7 and behind thermoplug 2. This thermalinsulator minimizes heat transfer between thermoplug 2, the surroundingwall of material specimen 3, and back cover 10. The thermal insulatoralso forces heat absorbed from front surface 4 of material specimen 3 tobe transferred nearly one-dimensionally along thermoplug 2.

The ends of thermocouple wires 11, e.g., Chromel-Alumel, are spot-weldedalong thermoplug 2, at various distances from front surface 4 ofmaterial specimen 3. Thermocouple wires 11 are routed through concentricannulus 7 to back surface 5 of the specimen material 3. Ceramic materialmay be placed between thermocouple wires 11 and the walls of materialspecimen 3 and thermoplug 2, thereby preventing the bare thermocoupledwires 11 from touching metallic parts. Thermocoupled wires 11 may beconnected, e.g., spliced, to larger diameter lead wire assemblies 12fastened to back surface 5 of material specimen 3.

Heat flux may be calculated from measured thermoplug temperatures usinga temperature variant thermal property inverse heat conductive problemmethod, e.g., heat storage equation: ##EQU1## In this equation, δT/δt isevaluated by differentiating least-squares curve fit equationsexpressing measured thermoplug temperatures as a function of time ateach temperature measurement location. Thermal properties are evaluatedat local temperatures measured on the thermoplug.

EXAMPLE

A plug-type heat flux gauge 1 according to the present invention wasformed in back surface 5 of a flat plate material specimen having athickness of 0.350 cm. Thermoplugs 2 were formed by electrical dischargemachining to have a diameter of 0.188 cm and a length of 0.330 cm.Chromel-Alumel thermocouple wires 11 were spot-welded at distances of0.00508, 0.0279, 0.175 and 0.330 cm measured from front surface 4 ofmaterial specimen 3. Lead wire assemblies 12 included Choromel-Alumelwires having a diameter of 0.015 cm encased in ceramic tubing. Theceramic tubing, in turn, was encased in Inconel® sheath material to forman assembly which was then swaged to size. The diameter of eachthermocouple wire 11 was 0.00762 cm. Each thermocouple wire 11 wasspot-welded to thermoplug 2, forming a cylindrical hot junction having adiameter of 0.0152 cm and a thickness of 0.00508 cm. The hot junctionclosest to the front surface 4 of material specimen 3 was welded to thebottom of a hole drilled through floor 8 of concentric annulus 7 along aline parallel to and 0.100 cm from the centerline of thermoplug 2. Thewidth of concentric annulus 7 was 0.080 cm. This hole was drilled at adepth of 0.00508 cm from front surface 4 of material specimen 3. Theother three hot junctions were welded to thermoplug 2 and werecircumferentially located 120° from each other. The thermocouple wires11 were extended from the hot junction in a direction perpendicular tothe surface on which they were welded, and then routed to lead wireassemblies 12.

As shown in FIG. 2, satisfactory agreement in both transient andsteady-state surface heat flux measurements between conventionalwater-cooled gauges and plug-type heat flux gauge 1 was obtained. An arclamp was used as the energy source. The arc lamp start-up time periodbetween about 0.3 and 0.7 seconds is defined in FIG. 2 as the region oftransient heat flux, while time after about 0.7 seconds is defined asthe region of steady-state heat flux. The timing of these regions wasconfirmed with a photodetector that simultaneously measured an increasein millivolt output of the arc lamp of about 3 to 54 MV at 0.3 to 0.7sec (128 MV/sec) after which the photodetector output remained constant.The round and square symbols represent the mean of three repeatedmeasurements taken with three separate thermocouple installations oneach plug-type heat flux gauge 1. The solid single line in the transientregion represents the cubic least-squares curve-fit relative to 100transient heat flux data points taken with the conventional water-cooledheat flux gauges. The mean of the steady-state heat flux data is alsoshown in FIG. 2.

Numerous modifications and adaptations of the present invention will beapparent to those so skilled in the art and thus, it is intended by thefollowing claims to cover all modifications and adaptations which fallwithin the true spirit and scope of the invention.

What is claimed is:
 1. A plug-type heat flux gauge, including anintegral thermoplug, comprising:a member having a first surface exposedto an energy source which heats said member and a second surfaceoppositely disposed from said first surface away from said energy sourcewith an annulus extending from said second surface toward said firstsurface, a thermoplug extending from said first surface to said secondsurface through said annulus with the surface of said thermoplug beingspaced from the surface of said annulus thereby forming a annularchamber, said thermoplug being integrally formed from a portion of saidmember in said annulus so that said thermocouple is of the same materialas said member whereby no seam connects the same, at least threethermocouple wires extending into said annular chamber from said secondsurface, each of said thermocouple wires being in heat conductingcontact with said thermoplug at a predetermined point, the spacingbetween each predetermined point and said first surface being differentfor each thermocouple wire, and means for covering said annular chamberat said second surface whereby said wires and contact points areinsulated.
 2. A plug-type heat flux gauge as recited in claim 1 whereinsaid thermoplug is cylindrical and said annulus surrounding saidthermoplug is concentric thereto.
 3. A plug-type heat flux gauge asrecited in claim 2, wherein said thermocouple wires are routed throughsaid concentric annulus.
 4. A plug-type heat flux gauge as recited inclaim 1, wherein:each of said thermocouple wires forms a hot junction ateach of said predetermined points.
 5. A plug-type flux gauge as recitedin claim 1 further comprising:a plurality of lead wires, each of saidlead wires having a diameter greater than that of a corresponding one ofsaid thermocouple wires and electrically connected to a second end ofsaid corresponding one of said thermocouple wires, said second end ofsaid thermocouple wires being opposite said first end.
 6. A plug-typeheat flux gauge as recited in claim 1 further comprising:a thermalinsulator within said concentric annulus and adjacent a first end ofsaid thermoplug; and a back cover secured to said material specimen andenclosing said thermoplug and said annulus and adjacent said first endof said thermoplug.
 7. A plug-type heat flux gauge as recited in claim6, wherein:said thermal insulator is a gaseous medium.
 8. A plug-typeheat flux gauge as recited in claim 1 wherein the means for covering theannular chamber comprises a back cover secured to the second surface.